An Overview on Short and Long-Term Response Energy Storage Devices for Power Systems Applications
Energy storage devices can be classified into short and long-term response, depending on their application. Technologies with high power density and with the ability to respond to the requests in short time fractions like flywheels, capacitors or superconducting magnetic coils belong to the so-called short-term response energy storage devices category. Energy storage devices with the capability to absorb and supply electrical energy for long periods of time like pumping hydro, batteries, compressed air and hydrogen fuel cells are considered in the long-term response category.
This paper concentrates on the latest short and long-term energy storage technology developments, performance analysis, and cost considerations.
Energy storage appears to be beneficial to utilities since it can decouple the instantaneous balancing between the demand and the supply. Therefore it allows the increased asset utilization, facilitates the penetration of renewable sources and improves the flexibility, reliability and efficiency of the grid .
However, the use of energy storage devices has not expanded significantly because of the state of technological developments and the price of energy storage devices which are still costly .
Nonetheless, there are several high performance storage technologies available today, or at an advanced state of development, which enables a new range of storage applications. For example, the issues related to the increasing integration of renewable sources in power systems have been one of the main drivers of this development.
Energy storage devices can be classified into two different categories, depending on their application: short-term response energy storage devices and long-term response energy storage devices.
Long-term response energy storage devices for power systems applications can usually absorb and supply electrical energy during minutes or hours and can specially contribute on the energy management, frequency regulation and grid congestion management , .
Short-term response energy storage devices are usually applied to improve power quality, particularly to maintain the voltage stability in power systems, throughout a contribution during transients (few seconds or minutes) .
2. Objectives and Methodology
An operation description of the available energy storage technologies will be presented followed by some application cases.
The main contribution of this paper comes from the technology comparison with special attention given to characteristics like power and energy range, efficiency, life-time and costs.
3. Long-Term Response Energy Storage Devices
The use of long-term energy storage devices is expected to rise in the next years because the generation availability fluctuations associated to the increasing integration of renewable sources in power systems .
Sort of different long-term response energy storage technologies are already available today. A brief description of these main devices is presented below.
A. Pumping hydro
The pumping hydro system set-up is presented in Fig. 1.
Restrictions to pumping hydro energy storage are related with geographical constraints and weather conditions. In periods of much rain, pumping hydro capacity can be reduced.
In Portugal, at 2006, the pumping-hydro installed capacity was 615 MW. This capacity had been used, during the last years, to store at the imported off-peak power from Spain.
Along this paper the distinction between two important battery concepts, electrochemical and redox flow, is emphasized.
Electrochemical batteries use electrodes both as part of the electron transfer process and store the products or reactants via electrode solid-state reactions .
There are a number of battery technologies under consideration for energy storage, where the main are:
· Lead acid
· Nickel cadmium
· Sodium sulphur
· Lithium ion
· Sodium nickel chloride
2) Redox Flow
Redox flow batteries are storage devices that convert electrical energy into chemical potential energy by charging two liquid electrolyte solutions and subsequently releasing the stored energy on discharge .
The name redox flow battery is based on the redox reaction between the two electrolytes in the system. These reactions include all chemical processes in which atoms have their oxidation number changed. In a redox flow cell the two electrolytes are separated by a semi-permeable membrane. This membrane permits ion flow, but prevents mixing of the liquids. Electrical contact is made through inert conductors in the liquids. As the ions flow across the membrane, an electrical current is induced in the conductors. 
Over the past few years three types of flow batteries were developed up to the stage of commercialization and demonstration. These types are vanadium cells (Vanadium Redox Batteries, VRB), Polysulphide Bromide Batteries (PSB) and zinc bromine (ZnBr). Each type has its own specifications and is developed for a specific application. 
This technology is suitable for energy storage application in the 5-500 MW range, operating in time fractions from 1 second to 12 hours. 
The first system based on Polysulphide Bromide Battery started to be built in Little Barford in the United Kingdom, with a combined-cycle power plant with the aim of load leveling. It was envisaged that came into operation in 2003, though some delays have led this unit does not come into operation and in 2004 the Regenesys stopped development of fuel cell type PSB. It was planned to this central a capacity of 120 MWh, with a peak power of 15 MW and more than 15 years of useful life, as well as efficiency between 60 and 65% with response time of less than 100 ms.
C. Compressed air
More often, the compressed air is mixed with natural gas and burnt together, in a conventional gas turbine. This method is actually more efficient as the compressed air will lose less energy.
The set-up of a compressed air system is presented in Fig. 3.
Three air reservoir types are generally considered: naturally occurring aquifers (such as those used for natural gas storage), solution-mined salt caverns, and mechanically formed reservoirs in rock formations. Mainly implementation constraints are related with reservoirs achievement .
The world's first CAES plant, the Huntorf plant, located in North Germany, was commissioned in 1978. This system has a total capacity of 290 MW, with storage capacity of approximately 600 MWh. The compressed air is used to actuate the turbines used for a nuclear power plant that is next. To be able to use this storage capacity up to two caves of salt-yolk with 500,000 m3 and 1,100,000 m3, store the air at a maximum pressure of 100 atm. 
D. Hydrogen fuel cell
A fuel cell is an energy conversion device that is closely related to a battery. Both are electrochemical devices for the conversion of chemical to electrical energy. In a battery the chemical energy is stored internally, whereas in a fuel cell the chemical energy (fuel and oxidant) is supplied externally and can be continuously replenished. 
The overall reaction in a fuel cell is the spontaneous reaction of hydrogen and oxygen to produce electricity in water. During the operation of a fuel cell, hydrogen is ionized into protons and electrons at the anode, the hydrogen ions are transported through the electrolyte to the cathode by an external circuit (load). At the cathode, oxygen combines with the hydrogen ions and electrons to produce water.
The hydrogen fuel cell system can be reversible, allowing electric power consumption for the production of hydrogen and that hydrogen can be stored for later use in the fuel cell. , 
The working scheme of a hydrogen fuel cell system is presented in Fig. 4.
Hydrogen volatility and its atoms reduced dimension put the hydrogen storage reservoir as the critical element in this device. Last research place Metallic Hydrates as one of most efficient , .
At moment, hydrogen fuel cell systems become one of the most referred storage technologies to set up renewable energy integration issue. Price and overall efficiency are its main constraints.
4. Short-Term Response Energy Storage Devices
Short-term response energy storage devices should be used to aid power systems during the transient period after a system disturbance, such as line switching, load changes and fault clearance. Their application prevents collapse of power systems due to loss of synchronism or voltage instability, improving its reliability and quality.
Short-term response energy storage devices use is getting common in power systems with important renewable energy penetration like wind and weak interconnections or in islands, avoiding temporary faults and contributing to the provision of important system services such as momentary reserves and short-circuit capacity. 
The main short-term energy storage devices and their operation are presented below.
At moment, in Portugal, there are two systems based on flywheels, implemented in Flores and Graciosa Islands, Azores. Over the past two years, EDA (Azores Electricity Company) worked with PowerCorp to maximize the wind penetration into the power system of these two islands. Thus, in March 2005, EDA and PowerCorp installed a 350 kW / 5 kWh flywheel device at Flores Island, where the generation consists in 2 wind turbines (15% of installed capacity), 4 diesel generators and 4 hydro-generators, totalizing 4.2 MW. Results obtained, in terms of network stability, encouraged the implementation of a similar system at Graciosa Island. 
Supercapacitors are electrochemical double layer capacitors that store energy as electric charge between two plates, metal or conductive, separated by a dielectric, when a voltage differential is applied across the plates. As like battery systems, capacitors work in direct current. This fact imposes the use of electronic power systems, as presented in Fig.6. 
C. Magnetic Superconducting
The SMES device set-up is presented in Fig. 7.
The performances of SMES offer very desirable benefits to power system applications. The first SMES power system application was proposed in 1969, with the objective of charging the superconducting magnet with the surplus generation of the basic load units during off-peak time, and discharge to the ac power system during peak time. In 1981, the first superconducting power-grid application was achieved. A SMES device for power quality and grid stability was located on the 500 kV Pacific Intertie that interconnects California and the Northwest, USA. This application demonstrated the viability of SMES to improve transmission capacity by damping inter-area modal oscillations. Since that time many studies and prototypes have been developed. , 
5. Energy Storage Devices Comparison
A summary of the main long-term response energy storage devices characteristics is presented in table I.
TABLE I - Long-term response energy storage devices characterization , , , 
Batteries and hydrogen fuel cell systems, on the other hand, are more modular and do not present so many physical restrictions to their implementation, being ideal for distributed storage.
Charge-discharge efficiency is one of most important issues for storage devices used in energy management. Hydrogen fuel cell system presents the lowest global efficiency, which results from its elements efficiency, since the hydrogen production till its use in the fuel cell. However, like this is an emergent technology, future efficiency improvements are expected.
Usually, high price and short life-cycle did not allowed a massive use of batteries in power systems, however redox flow batteries introduced a new dynamic in these storage solutions and present an important development potential, specially for the grid integration of variable renewable energies like wind.
Short-term response energy storage devices characterization is presented in table II.
Supercapacitors are the most compact short-term energy storage devices, use the simplest operation apparatus and are virtually maintenance free.
Flywheels present a higher range, in terms of energy storage capacity. They are also cheaper than supercapacitors for high energy capacity. Because mobile mechanical component and to avoid human and material hazardous, flywheels operation requires some special maintenance and safety concerns.
6. Conclusions and Perspectives
In this regard, this paper presents an overview on energy storage devices for power systems applications in the framework of a broader project that intends to project an energy storage system for facilities based on non-dispatchable renewable energies.
From the analysis performed, it is concluded that long-term energy storage devices like pumping-hydro and compressed air systems are the best suited for centered large-scale storage, on the other hand, batteries and hydrogen fuel cell systems space requirements and modularity place them as ideal solution for distributed energy storage.
Moreover, short-term response energy storage devices like supercapacitors are found to be well suited for use in power systems during transient periods that result from a system disturbance such as a line switching. Flywheels, with higher energy storage capacity look like the most appropriate to maintain voltage level and frequency, especially in power systems with considerable penetration of renewable energy like wind.
Further developments will focus on the dimensioning and development of an energy storage system for a Portuguese wind farm where short and long-term technologies will be combined.
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